Union leaders are urging the government to end uncertainty over a new nuclear power station in Suffolk, to secure thousands of jobs.
The GMB said reports that ministers are considering alternatives to the involvement of a Chinese company in building Sizewell C risked the loss of huge numbers of jobs.
Funding needs to be in place for the construction phase of the project by the autumn, or the whole scheme is at risk of collapsing, according to the union.
China's state-owned energy company CGN is part of a consortium behind the planned new plant in Suffolk.
GMB national officer Charlotte Childs said: "If ministers are set to block China's involvement, they need to tell us who is going to pay for it.
"We need urgent clarity on how the funding mechanism will work by the autumn, otherwise the whole project could collapse.
"The Government must step in with a credible alternative to CGN funding that creates a low risk and affordable investment model for third parties, taking a public stake in key projects that will create a clear path to net zero.
"If there is a change in policy then our members in construction will deliver a world-class project, but the Government needs to end the uncertainty, back this essential infrastructure and start investing in our own manufacturing supply chain."